This system uses "Track Circuit Detection" to detect trains on sections of track. This requires that the rails in each section of track be insulated from each other (I will leave the details of track circuit detection and dealing with wet/dry track, wheel noise, etc. for another time).
The system works well but, there are many tracks that use steel bar stock with welded steel ties/gauge bars or similar construction where the rails are not insulated from each other and thus track circuit detection of trains is not possible/practical.
I know that there are many tracks that use engineer operated buttons, toggles, etc. to control signals but here I am interested only in Fully Automatic Signals.
There are methods of detecting trains entering & leaving a block that can be used in these situations, some are:
- Short sections of insulated rail to detect axle shunts
- Counting wheels
- Mechanical (micro-switches, etc.)
- Optical sensors
- Proximity sensors (assumes ferrous wheels)
- They are subject to activation by pedestrians, animals, etc. and, in the case of optical sensors, falling leaves (which can also block optical sensors).
- All of these methods need to be able to distinguish direction
- They must handle the case where half of a train passes over the sensor(s), stops, and then backs up - thus possibly resulting in a count error (i.e.: wheel stopped on top of a sensor).
- They must be able to handle extremely short trains: e.g.: A "peddle car", "Motor Car" or other 2-axle vehicle.
The problem with "wheel Counting" in general is that the system must handle the situation where there is a 'count error' where, after the train leaves the wheel count does not go to zero because of a counting error going in or out.
There is also the case where, half way through the block a car failed and was set off the track. The train then exits the block and it appears that a car is still in the block - thus the block is still marked as occupied.
Many who try this handle the 'count error' situation by having a timer on the track occupancy - i.e.: if the track has been occupied for longer than, lets say, 5 minutes the system will "assume" that the train left and that there was a count error and thus it will clear the block. That's fine in many cases but do you want to be working on your derailed locomotive and have someone come around a blind curve at you? not me ...
There are full size railroads that use wheel counting but their sensors are much more accurate, durable, ... and more expensive ... than what the average hobby railroad would use, and, if they do get a miss-count, the block remains out of service until another train traverses the block at dead-slow speed while in constant contact with a human dispatcher. After that, the block is cleared by the dispatcher. This does not seem like a procedure most hobby railroads would tolerate.
Some tracks will use a detector on the ENTRY and EXIT tracks of a block and assume (there's that word again) that, when a train trips the ENTRY detector he will not back up ... same as the miss-count situation, the block remains occupied but there is no train in the block. Also, this only works in simple passing siding situations and does not work where a block contains ENTRY points that are also EXIT points.
So, my question is, has anyone seen an approach to Fully Automatic Signals that does not use track circuit detection and deals with welded ties?